DC-to-DC power converters are widely used to supply power to electronic devices, such as in computers, printers, and other devices. Such DC-to-DC converters are available in a variety of configurations for producing the desired output voltage from a source voltage. For example, a buck or step down converter produces an output voltage that is less than the source voltage. A typical step down converter includes one or more power switches which are pulse width modulated to connect the source voltage to an output inductor to thereby power the load.
For example, the HIP5061 converter offered by the assignee of the present invention is a complete power control integrated circuit incorporating both the high power DMOS transistor, CMOS logic and low level analog circuitry on the same IC. The converter includes a gate driver for the high side switch, and a high speed peak current control loop. A portion of the converter's DC output is applied to a transconductance error amplifier that compares the fed back signal with an internal reference. The feedback signal is generated by a resistor divider connected across the output of the converter.
The output of the error amplifier is also brought out at a terminal to provide for soft start and frequency compensation of the control loop. This same signal is applied internally to program the peak high side switch drain current. To assure precise current control, the response time of the peak current control loop is less than 50 ns.
The transconductance error amplifier compares the DC level of the fed back voltage with an internal reference, while providing voltage loop compensation using external resistors and capacitors. The error amplifier output is converted into a current to program the required peak high side switch current that produces the desired output voltage. When the sum of the sensed high side switch current and the compensating ramp exceed the error current signal, a latch is reset and the high side switch is turned off. Current comparison around this loop takes place in less than 50 ns, thereby allowing for excellent 250 KHz converter operation.
A signal proportional to the output inductor's current may be used to limit component stress during output overloads (overload protection). However, a regulation application needs a higher fidelity current signal than the overload protection application. Typically the heat sinks and thermal design of the DC-to-DC converter are sized for efficiency, and the worst case variation of the overload trip level (current signal) still maintains the components below their maximum ratings. Unfortunately, the waveshape may not be suitable for regulation, and any sensing circuit bandwidth must be sufficient in view of the switching frequency.
Perhaps the most common approach to sensing the output inductor current in a buck converter uses a sensing resistor connected in series with the output inductor. The circuit reconstructs the output inductor current as a differential voltage across the sensing resistor. Most IC's using this approach regulate the output voltage with current mode control and use the signal for output voltage feedback.
The sensing resistor value must be large enough to keep the sensed signal above the noise floor and yet small enough to avoid excessive power dissipation. This approach has the obvious efficiency drawback with high output current. In other words, power is unnecessarily dissipated by the sensing resistor, especially since the power dissipated increases with the square of the inductor current. For some applications, the value of the sensing resistor may be close to the same resistance as the MOSFET's on resistance.
In another variation of the sensing circuit, the sensing resistor is connected in series with the drain of the upper MOSFET as disclosed, for example, in the MAXIM MAX1624/MAX1625 data sheets. This has the advantage of reducing the power dissipated in the sensing resistor with a large signal (large resistor value).
Unfortunately, this sensing resistor location creates other problems that can lead to a less robust design. The main problem is that the upper MOSFET's drain current is discontinuous. Every time the upper MOSFET turns on, the current starts at zero and increases rapidly with a steep slope. Additionally, the current waveshape exceeds the inductor current for the time interval necessary to replenish the charge of the lower MOSFET and/or diode junction capacitances. The control IC must first ignore the additional initial current and wait for the signal to settle. This may cause control loop problems and limit the input-to-output range of the converter.
Yet another approach of current sensing is illustrated by the HIP6011 converter offered by the assignee of the present invention. This converter uses the upper MOSFET's on-resistance as the current sensing element for overload protection. This IC uses voltage-mode control for output voltage regulation and can tolerate the large variation in the resistance value. However, a droop circuit may be needed that modifies the output voltage as a function of load current. The droop circuit uses the average voltage drop across the output inductor (a resistor and capacitor as a low-pass filter) to modify the output voltage regulation. The average voltage across the inductor is the DC output current multiplied by the inductor's winding resistance.